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Evolution speeds up with help from microorganisms
The acceleration of the speed and complexity of evolution has until now remained a mystery.
The fossil record confirms that single-celled life first appeared about 3.5 billion years ago, and it took another 2.5 billion years for multi-cellular life to evolve. In roughly 1 billion years, however, plants, mammals, insects, birds and other species evolved and populated the earth.
New studies by Rice scientists suggest that evolution has speeded up because bacteria and viruses constantly exchange transposable chunks of DNA between species, making it possible for life forms to evolve faster than if they relied exclusively on sexual selection or random genetic mutations.
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| Michael Deem, the John W. Cox Professor in Biochemical and Genetic Engineering and professor of physics and astronomy |
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"We have developed the first exact solution of a mathematical model of evolution that accounts for this cross-species genetic exchange," said Michael Deem, the John W. Cox Professor in Biochemical and Genetic Engineering and professor of physics and astronomy.
The research appeared in the Jan. 29 issue of Physical Review Letters.
Past mathematical models of evolution have focused on how populations respond to point mutations—random changes in single nucleotides on the DNA chain, or genome. A few theories have focused on recombination—the process that occurs in sexual selection when the genetic sequences of parents are recombined.
Horizontal gene transfer (HGT) is a cross-species form of genetic transfer. It occurs when the DNA from one species is introduced into another. The idea was ridiculed when first proposed more than 50 years ago, but the advent of drug-resistant bacteria and subsequent discoveries, including the identification of a specialized protein that bacteria use to swap genes, has led to wide acceptance in recent years.
"We know that the majority of the DNA in the genomes of some animal and plant species—including humans, mice, wheat and corn—came from HGT insertions," Deem said. "For example, we can trace the development of the adaptive immune system in humans and other jointed vertebrates to an HGT insertion about 400 million years ago."
The new mathematical model developed by Deem and visiting professor Jeong-Man Park attempts to define how HGT changes the dynamics of evolution. In comparison to existing models that account only for point mutations or sexual recombination, Deem and Park's model shows how HGT increases the rate of evolution by propagating favorable mutations across populations.
Deem described the importance of horizontal gene transfer in a January 2007 cover story in the Physics Today, showing how HGT compliments the modular nature of genetic information, making it feasible to swap whole sets of genetic code - like the genes that allow bacteria to defeat antibiotics.
"Life clearly evolved to store genetic information in a modular form, and to accept useful modules of genetic information from other species," Deem said.
The research is supported by the Defense Advanced Research Projects Agency.
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